1. Field of the Invention
The present invention relates to inertial navigation systems. More particularly, though not exclusively, the present invention relates to an inertial navigation/guidance system using a radio navigation receiver to correct the navigation errors.
2. Problems in the Art
Differential global positioning system (DGPS) based guidance systems for airborne application of agrochemicals has met with huge success and it follows that this technology could be used with ground rig applicators if certain technical problems can be overcome. The primary differences between air and ground application processes are associated with the operators view of the land and the operational dynamic environment.
During airborne applications, the pilot generally has a large part of the area involved to be sprayed in view and the GPS antenna (mounted on top of the canopy) follows a relatively straight line when in an application swath. This provides for the required cross track position stability to obtain a well controlled application process.
With ground rig applicators, such as tractors or floaters, the operator may have a limited view of the involved spray area and depending on the size/shape of the field and of the local ground cover it can be very difficult to determine where the previous swath coverage ends in order to proceed with the ensuing swath. A GPS antenna mounted on top of the ground vehicle (where it would be exposed to the GPS satellites) will experience large attitude excursions as the rig swaths the field. This results in GPS derived cross track position excursions relative to the vehicle ground track which would contaminate any attempt to parallel a defined field line. It can therefore be seen that there is a need for a better navigation/guidance system for use with a ground-based vehicle.
There is a need for a solution to various problems relating to ground rig applicators such as tractors and floaters. These ground rig applicators have several disadvantages. Since the application of agrochemicals is a seasonal process (3-4 months per year), the workers hired to operate the floaters are seasonal workers. As a result, the seasonal workers are often inexperienced or unreliable in the operation of the floaters. This increases the probability that the operator will skip areas of the field or apply double coverage to overlapping areas. These problems cost the seller of the chemicals and the farmer thousands of dollars. A typical floater will cost $100,000 and will apply $500,000-$1 million dollars worth of chemicals per year. It can therefore be seen that efficient operating of a floater is very important. Typical prior art floaters are guided through a field by following a trail of foam markers which are marked on the field on the previous swath. As a result, there is a lot of room for human error and the floaters cannot be operated at night. A need can therefore be seen which would allow the floaters to operate accurately day or night throughout the season. Such a solution would allow a chemical applier to operate half as many machines. An accurate, real-time inexpensive navigation/guidance system would remedy these problems.
Various prior art navigation systems for ground-based vehicles have several disadvantages. Systems using Doppler radar will encounter errors with the radar. Similarly, gyros will encounter drift errors which will continue to increase unless the drift error is corrected. Gyros that are inexpensive enough to be feasible to use may have drift rates high enough to make them unusable. For example, a typical inexpensive gyro sensor may have a drift rate uncertainty as high as 3600.degree. per hour which makes the gyro unusable for most applications. As a result, gyros have good short term characteristics but bad long term characteristics as the drift error becomes larger as time goes on.
When navigating using dead reckoning you need a very high fidelity heading measurement which has not been achieved adequately using low costs sensors.
Various prior art systems navigating by GPS also have disadvantages. Prior art systems using GPS use GPS as the primary navigator. This intensifies the problems found with GPS. A GPS position calculation has a lag time. As a result, the position solution provided by a GPS receiver tells a user where the vehicle was a moment ago not in real time. Another problem with GPS systems are the errors resulting from the antenna lever arm problem. A GPS antenna typically is a certain distance away from the GPS receiver. Since the GPS antenna is the collection point of the GPS signals received, the position solution will not accurately describe the position of the GPS receiver or some other reference point. If the geometrical distance between the GPS receiver or reference point and the GPS antenna is known, the position of the reference point may be calculated. However, as a ground based vehicle travels over uneven terrain such as terraces, slopes, ruts, bumps, etc., the actual position of the GPS antenna cannot be determined resulting in erratic GPS position solutions.
Most prior art attempts to use a GPS navigation system attempted to deal with problems by correcting GPS drift and lag time. However no prior art system navigating by GPS has achieved the high accuracy and real time solutions required for applications requiring a high level of accuracy. The prior art attempts have not provided an adequate solution because GPS does not provide a continuous navigation solution. A GPS system will update its position periodically, not in real time, and a lag time is still involved. Another problem with a GPS system is the possibility of a signal dropout of the satellite signals. The accuracy of a GPS system is also limited due to the errors caused by the ionosphere. Another problem with GPS systems is that altitude data provided by a GPS receiver is not precise.
Another problem with GPS systems is that a GPS system cannot accurately supply guidance data for a curved path. This problem relates to the lag time involved with GPS. A GPS system can only do linear interpolation of GPS position solutions which is inadequate for navigating a curved path. A GPS system also will not provide high quality heading information. Finally, the altitude calculated by a GPS receiver is inaccurate and unusable for certain applications.